elvis
Well-known member
The concept of "hot" and "cold" bias is amp tech voodoo. Tubes are biased primarily so that they stay pretty linear as they deliver the intended power to the speaker. There are a range of bias conditions that control this, including plate voltage and current. Usually the circuit is designed by drawing a load line on the tube characteristics listed in the datasheet, and finding a reasonable area in the curves. Biasing with high voltage and/or current dissipates more power than low voltage and/or current, and each affects both lifetime and linearity. There is usually a range of voltages and currents that give a decent combination of life, linearity and power. The bias can also affect crossover distortion, though this is often more a characteristic of tube and transformer tap matching.
The range of proper bias is usually not terribly large in the sense that you can detect it with an ammeter, but not with your eye. If you bias based on the color of the arc in the tube, you are way outside of the specified bias range of the tube.
If changing bias makes you like the sound better, go for it. But be aware that you may be trading tube life or amp life. Dissipating more power in the tube makes it die quicker, and more likely to supernova, taking some amp components with it.
A manufacturer who wants a reputation that includes reliable amp performance will be careful to bias tubes so that they are not pushed terribly hard, but still give the rated power output. If you don't like that, fine - change it. But don't get mad at the company, and be aware that you are almost always trading reliability for whatever you feel you get from rebiasing the amp.
There is no single "ideal" or "correct" bias point. It's all tradeoffs.
The range of proper bias is usually not terribly large in the sense that you can detect it with an ammeter, but not with your eye. If you bias based on the color of the arc in the tube, you are way outside of the specified bias range of the tube.
If changing bias makes you like the sound better, go for it. But be aware that you may be trading tube life or amp life. Dissipating more power in the tube makes it die quicker, and more likely to supernova, taking some amp components with it.
A manufacturer who wants a reputation that includes reliable amp performance will be careful to bias tubes so that they are not pushed terribly hard, but still give the rated power output. If you don't like that, fine - change it. But don't get mad at the company, and be aware that you are almost always trading reliability for whatever you feel you get from rebiasing the amp.
There is no single "ideal" or "correct" bias point. It's all tradeoffs.
